The present invention relates to a process for the control of an optical characteristic of a material. It is used in optoelectronics and mainly in the control of liquid crystal cells used more particularly as converters of electrical data into optical data, in the real time processing of optical images, in the formation of coloured filters which can be used in colour television and for analog display purposes.
Although it applies to materials of random shapes and sizes, the invention more particularly relates to the control of an imager comprising a display cell made from a material with a plurality of zones distributed in matrix-like manner and intercalated or interposed between a so-called cross-bar system. Such systems consist of a first group of p lines of parallel electrodes and a second group of q columns of parallel electrodes, the lines and columns being crossed. A zone x.sub.i y.sub.j of the material is defined by the overlap region between line x.sub.i (in which i is an integer which can assume all values between 1 and p) and the column y.sub.j (in which j is an integer between 1 and q). The bars formed by the lines and columns of electrodes are such that they can carry appropriate signals for the excitation of the material.
Numerous devices of this type are known which, for example, use as the sensitive material a liquid crystal film and for which excitation is of an electrical nature. The invention more particularly applies to such devices, but it applies in a more general manner to any cross-bar device made from a material, whose optical characteristic can be modified by a random excitation. This excitation can be of an electrical nature, as for liquid crystals, but may also be magnetic, thermal, etc. The material can be a solid, liquid, amorphous or crystalline substance. The optical characteristic can be an opacity, a refractive index, a transparency, an absorption, a diffusion, a diffraction, a convergence, a rotatory power, a birefringence, an intensity reflected in a given solid angle, etc.
Besides liquid crystals materials which can be used are, for example, cadmium sulphide crystals which have an absorption band, whose front can be displaced by a thermal effect. In this case the controllable optical characteristic is the absorption of light and the excitation is a heating action. Reference can also be made to the case where the optical characteristic is the intensity of light reflected by a deformable diaphragm and in which the excitation is an electrostatic force.
A known control process comprises, for example, in the case of a liquid crystal cell where the excitation is of an electrical nature, the application to line x.sub.i of a sinusoidal voltage V.sub.x and to the other lines a zero voltage and the application to the columns y.sub.j of sinusoidal voltages V.sub.yj of the same duration and frequency as voltage V.sub.x, but they are displaced by .phi..sub.ji with respect to voltage V.sub.x. This phase displacement .phi..sub.ji makes it possible to vary the intensity of the signal to be displayed, i.e. makes it possible to obtain different grey levels.
Such a control process is described in French Patent No. 2 279 123, filed on Feb. 6, 1974 by the present Applicant entitled "Process for the control of an optical characteristic of a material and analog imager utilizing this process".
To obtain N grey levels with such a process, it is necessary to produce N different voltages V.sub.yj. To apply one of the N voltages V.sub.yj to each of the q columns without producing them on each integrated circuit chip, it is necessary to use N transmission gates per column, i.e. q times N gates and to provide N connections per chip.